This invention relates to a method of starting a starting electric motor used for starting an internal combustion engine and, more particularly, to a method of starting a magnet type of starting motor.
In general, for starting an internal combustion engine such as utilized in automotive vehicles, a starting electric motor such as battery-driven permanent magnet type D.C. motor is used. The motor is made up of a cylindrical yoke, a permanent magnet secured to the inner peripheral surface of the yoke, an armature mounted in said yoke in a facing relation to said permanent magnet and provided with an armature winding, a commutator mounted on a rotary shaft provided on said armature and electrically connected to said armature winding, brush means slidingly contacting the peripheral surface of said commutator and supplying current to said armature winding, an overrunning clutch engaged with and movable axially of said armature and a pinion operatively associated with one side of said overrunning clutch.
When the starting motor is driven to rotate, the pinion mounted on the output shaft of the motor through the medium of the overrunning clutch is set into rotation and the internal combustion engine is started upon actuation of a shift lever whereby the pinion is brought into mesh with a starting ring gear provided on the engine.
In conventional starting motors, positive and negative brushes are stationarily mounted within the motor housing or yoke so that, when an exciting current is supplied to the armature winding, the slide contact points of the brushes are situated on a mechanical neutral point and an electrical neutral axis, this brush position being most preferred theoretically in order that a maximum electric output may be developed by the starting electric motor.
With the above construction of the conventional starting motor, a rush current flows through the armature at the instant the pinion is engaged with the ring gear. The result is that a sudden impact load is produced in the torque transmission system. In effect, the pinion is engaged with the ring gear while the latter is stationary, and the rotational speed of the starting motor is increased from zero speed with the pinion. Thus the rush current flows through the armature winding depending on the characteristics of the starting D.C. motor. As a result, the maximum rush current flows at the instant the pinion is engaged with the ring gear so that the generated magnetic flux in the armature winding and through the permanent magnet are in the reverse direction to the main magnetic path in accordance with the right-hand rule. In this manner, the magnetic field of the permanent magnet tends to be weakened through demagnetization.